Citrus canker, caused by the Gram-negative bacterial pathogen Xanthomonas citri subsp. citri (Xac) (syn. Xanthomonas citri, X. campestris pv. citri and X. axonopodis pv. citri) (Schaad et al., 2006), is one of the major constraints to citrus production worldwide (Gottwald et al., 2002). Citrus canker affects most commercial citrus varieties and is widely distributed in many tropical and subtropical citrus growing regions (Gottwald et al., 2002, Graham et al., 2004). The disease characterizes itself as raised necrotic lesions surrounded by oily, water-soaked margins and yellow chlorotic rings on leaves, stems and fruit of infected trees; and when conditions are highly favorable for disease, it also causes defoliation, twig dieback, general tree decline, blemished fruit and premature fruit drop (Gottwald et al., 2002, Graham et al., 2004). Wind-blown rain, which facilitates invasion via stomata or wounds (Graham et al., 2004), is the primary short distance (i.e., within the same tree or between neighboring trees) spread mechanism for citrus canker (Gottwald et al., 2009). Severe meteorological events such as hurricanes and tornados can disseminate the disease over longer distances (key et al., 2006; Gottwald and key, 2007). However, long-distance dissemination more often occurs with the transportation of diseased propagating materials, e.g. budwood, rootstock seedlings, or budded trees, and infected fruit (Gottwald et al., 2009). Direct canker-related losses are attributed to the decrease of fruit quality and yield. Moreover, a serious consequence is the significant impact on commerce resulting from restrictions to national and international fruit trade from canker affected areas (Gottwald et al., 2002; Shiotani et al., 2009).
Copper-based bactericides are currently the primary control measure for citrus canker worldwide, as limited strategies exist for suppression of citrus canker on susceptible cultivars once the pathogen has been established (Graham et al., 2004). For effective disease control, multiple applications of copper-based bactericides are needed throughout the year due to their partial effectiveness under windblown rain conditions (Behlau et al., 2010b; Graham et al., 2010). As a consequence, long-term use of copper bactericides led to induced resistance to copper in xanthomonad populations (Ritchie and Dittapongpitch, 1991; Behlau et al., 2012) and the accumulation of copper metal in soils or ground water affecting the environment and plant health (Alva et al., 1993). Thus, there exists a need to develop novel strategies to deal with this situation. A potential approach is to develop compounds that reduce bacterial resistance to copper bactericides and, thereby enhancing the bactericidal effect and reducing the application of copper.
Much effort has been made to understand the biology and molecular basis of Xac pathogenesis over the past decade (da Silva et al., 2002; Brunings and Gabriel 2003; Laia et al, 2009; Yan and Wang, 2012). The canker bacterium, considered to be a hemibiotrophic pathogen, initially grows epiphytically (on leaf surfaces) and then enters into the host through stomata or wounds to colonize the mesophyll parenchyma and multiply in the apoplast (intercellular spaces). Xac, like many other plant pathogenic bacteria, has evolved multiple virulence factors to promote infection and establish themselves successfully in host plants (Laia et al, 2009; Yan and Wang, 2012). Among the virulence factors important for Xac infection, biofilms have been suggested to play an important role in the early stages of infection by enhancing epiphytic persistence on host leaves. Importantly, multiple mutants of Xac impaired in biofilm formation consistently exhibit a decrease in bacterial growth in planta and have reduced ability to elicit canker symptoms in susceptible citrus leaves (Gottig et al., 2009; Guo et al., 2010; Rigano et al., 2007; Li and Wang, 2011; Yan and Wang, 2011).